Embodiments of the invention relate generally to positron emission tomography (PET) and magnetic resonance (MR) imaging, and more specifically, to a combined PET-MR system incorporating a stationary anterior phased array surface coil that can be positioned stationary relative to a patient or patient table, with the coil being constructed so as to enable isolation of the coil from the patient and so as to minimize any affect on PET-MR image acquisition.
PET imaging involves the creation of tomographic images of positron emitting radionuclides in a subject of interest. A radionuclide-labeled agent is administered to a subject positioned within a detector ring. As the radionuclides decay, positively charged photons known as “positrons” are emitted therefrom. As these positrons travel through the tissues of the subject, they lose kinetic energy and ultimately collide with an electron, resulting in mutual annihilation. The positron annihilation results in a pair of oppositely-directed gamma rays being emitted at approximately 511 keV.
It is these gamma rays that are detected by the scintillators of the detector ring. When struck by a gamma ray, each scintillator illuminates, activating a photovoltaic component, such as a photodiode. The signals from the photovoltaics are processed as incidences of gamma rays. When two gamma rays strike oppositely positioned scintillators at approximately the same time, a coincidence is registered. Data sorting units process the coincidences to determine which are true coincidence events and sort out data representing deadtimes and single gamma ray detections. The coincidence events are binned and integrated to form frames of PET data which may be reconstructed into images depicting the distribution of the radionuclide-labeled agent and/or metabolites thereof in the subject.
MR imaging involves the use of magnetic fields and excitation pulses to detect the free induction decay of nuclei having net spins. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In MRI, it is desirable for the excitation and reception to be spatially uniform in the imaging volume for better image uniformity. In a standard MRI system, the best excitation field homogeneity is usually obtained by using a whole-body volume RF coil for transmission. The whole-body transmit coil is the largest RF coil in the system. A large coil, however, produces lower signal-to-noise ratio (SNR) if it is also used for reception, mainly because of its greater distance from the signal-generating tissues being imaged. Because a high signal-to-noise ratio is the most desirable in MRI, “surface coils” are commonly employed for reception to enhance the SNR from a particular volume-of-interest. Such surface coils are relatively small and are constructed to receive the MR signal from a localized portion of the patient. For example, different surface coils may be employed for imaging the head and neck, legs and arms, or various internal organs. One particular type of surface coil that is often employed is an anterior array surface coil that is used to image a region-of-interest located in an anterior portion (i.e., a frontal portion) of the patient.
Currently, the industry standard is to place the anterior array surface coil on top of the patient, such that it rests on the patient. As a result, this surface coil moves as the patient breaths or moves. However, this movement of the surface coil creates a number of challenges with regards to a PET-MR hybrid system, namely because the positioning of ancillary devices on the patient (e.g., surface coils) has never been a requirement for PET imaging since conventional PET imaging systems eliminate all random-placed accessories relative to the patient to reduce artifact. In employing a surface coil for acquiring MR image data during a PET-MR imaging acquisition, the motion of the surface coil makes the attenuation correction process challenging, as there would be a need to know the position of surface coil real-time during imaging. And while patient motion correction is not new to either PET or MR, the combination of the two imaging modalities in one simultaneous process creates completely unique challenges. Namely, conventional “patient breath-holds” or special MR scan sequences that are often employed for stand-alone MR imaging will not be adequate for PET-MR imaging.
It would therefore be desirable to provide a surface coil for use in a PET-MR system that can be positioned stationary relative to a patient or patient table, such that movement of the coil that might be caused by patient movement is eliminated. It would also be desirable for the surface coil to have a construction that minimizes any affect on PET-MR image acquisition, such as by forming the surface coil from a material having a low proton density and material density.